Explosive echo ranging device

ABSTRACT

A sonic-pulse-producing assembly for use in underwater echo ranging which comprises A. A LINEAR CHANGE OF DETONATING EXPLOSIVE ARRAYED IN THE FORM OF A HELIX HAVING A UNIFORM PITCH, P, which satisfies the relationship WHERE D is the detonation velocity of said explosive, L is the length of explosive in each turn of said helix, and C is the velocity of sound in sea water, said linear charge being so arrayed that the closest approach of explosive in successive turns of said helix is no less than the minimum cross-sectional dimension of said linear charge; B. SUPPORTING MEANS ADAPTED TO MAINTAIN SAID EXPLOSIVE IN SAID HELICAL ARRAY; AND C. AN INITIATOR IN INITIATING RELATIONSHIP TO ONE END OF SAID EXPLOSIVE, SAID INITIATOR BEING ADAPTED TO BE ACTUATED BY A SELF-CONTAINED ACTUATING MEANS WHEN SAID ASSEMBLY IS AT A PREDETERMINED DEPTH IN THE OCEAN.

United States Patent Andrews et al.

[54] EXPLOSIVE ECHO RANGING DEVICE [72] Inventors: Adlay B. Andrews,Woodbury, N.J.; David L. Coursen, Newark, DeL; Frank A. Loving Jl'-,Wenonah, NJ.

[73] Assignee: E. I. du Pont de Nemours and Company,

Wilmington, Del.

[22] Filed: Apr. 23, 1963 [21] Appl. No.: 275,464

Related US. Application Data [63] Continuation-in-part of Ser. No.l3,385, Mar. 7, 1960,

2,953,092 9/1960 Walker ..l02/7 [151 3,656,585 [451 Apr. 18,1972

Primary ExaminerRobert F. Stahl Attorney-Samuel S. Blight [57] ABSTRACTA sonic-pulse-producing assembly for use in underwater echo rangingwhich comprises a. a linear charge of detonating explosive arrayed inthe form of a helix having a uniform pitch, P, which satisfies therelationship PsOJSXC/DXL where D is the detonation velocity of saidexplosive, L is the length of explosive in each turn of said helix, andC is the velocity of sound in sea water, said linear charge being soarrayed that the closest approach of explosive in successive turns ofsaid helix is no less than the minimum cross-sectional dimension of saidlinear charge;

b. suppgrting-means adapted to maintain said explosive in said helicalarray; and c. an initiator in initiating relationship to one end of saidexplosive, said initiator being adapted to be actuated by aself-contained actuating means when said assembly is at a predetermineddepth in the ocean.

5 Claims, 17 Drawing Figures PATENTEDAPR 18 m2- sum 10F 7 INVENTORSALDAY B. ANDREWS DAVID L. COURSEN FRANK A. LOVI NG,JI.

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PATENTEMRHMQYZ 3,655,585

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PATENTEUAPR 1a 1912 ALDA YYYYYYYY WS D A V I D L C 0 U R S ENPATENTEDAPR 18 I972 SHEET 5 BF 7 INVENTORS ALDAY B. ANDREWS DAVID L.COURSEN BY FRANK A. LOV|NG,Jr.

PATENTEUAPRMIQ 3,656,585 SHEET 6 BF 7 INVENTORS ALDAY B. AN DREWS DAVIDL.'COURSEN FRANK A. LOV|NG,Jr.

:PATE'N'TEDAPR 18 I972 SHEET- 7 OF 7 Frequncy INVENTORS ALDAY B.ANDREWSDAVID L.COURSEN FRANK A.LOVINGJ[ EXPLOSIVE ECHO RANGING DEVICE Thepresent application is a continuation-in-part of our prior applicationSer. No. 13,385, filed Mar. 7, 1960, now abandoned.

The present invention relates to an improved device for echo ranging.More particularly, this invention relates to an explosive sonic sourceemitting a sequence of pressure pulses having a predetermined Dopplercharacter.

The acoustic detection of submarines is based on the transmission ofsound from the submarine to the detector element located in the oceanarea in which the submarine is operating. The sound emanating from thesubmarine can be either noise generated internally within the submarine,noise caused by the submarine passing through the water, or sound(echoes) reflected by the submarine from nonconnected sound sources inthe ocean. Active acoustic systems, which use nonconnected soundsources, are particularly useful for detecting quiet submarines.

The sonic sources generally employed for underwater echo ranging areelectromechanical transducers and explosives. Explosives are moreadvantageous in several respects: (1) they generate a high-energyacoustic signal in useful frequency bands and thus are effectiveoverlong distances; (2) they can be placed conveniently at great depths inthe ocean where the troublesome effects of refraction on soundtransmission to the target are minimized; and (3') because of theirlight weight and independence of electrical power sources, they aresuitable for launching from fixed-wing aircraft.

The explosive charge which has heretofore been employed in echo rangingis a point charge which emits a single pulse of acoustic energy whendetonated. This acoustic source is used in combination with anomnidirectional hydrophone as the acoustic receiver. The usefuldetection range of such a system is limited primarily by reverberation(i.e., the reflected and scattered energy received from the bottom,surface, and volume of the ocean) because the sound generated bydetonation of the charge is radiated with equal intensity in alldirections. The acoustic energy reflected and scattered from the oceansurface and bottom toward the hydrophone will often obscure the energy(echo) reflected from a submarine at ranges which equal or exceed theocean depth.

An explosive charge in the form of a line, a so-called line charge isbeing introduced for use in explosive echo ranging systems to providesome control over the direction in which the major portion of theacoustic energy is emitted by an explosive charge and thereby to reducethe reverberation problem. Like the point charge, the line charge alsoemits a single pulse of acoustic energy. However, when a line charge isoriented vertically in the ocean and detonation is initiated at thebottom end, most of the energy is emitted in an acoustic beam with itsaxis about 13: above the horizontal. The form of this beam patterndepends upon the length of the line charge, longer charges givingnarrower beams, and lower vertical side lobe levels. The narrow beam andlow vertical side lobe level from a long line charge mean that lessenergy is radiated toward the ocean surface and bottom especially in thenear vertical direction from the charge. Thus, if a submarine lies inthe path of the beam from a line charge, there will be less acousticenergy scattered back from the surface and bottom and therefore therewill be less undesirable interference with the echo of the submarinethan in the point charge case. An obvious disadvantage of the narrowerbeam is that the submarine must lie in a relatively small volume ofocean if it is to be detected; another disadvantage is that the sound isscattered by ocean surfaces at about the same range and elevation angleas the submarine target. If the single beam lobe of the line charge istoo narrow, the probability of finding a submarine will be less.

Thus, point charges and line charges are of limited use for echo rangingbecause of the reverberation problem with point charges, and thedecreased probability of finding a target within the narrow beam fromline charges which are suffrciently long to suppress the reverberation.

The present invention provides a novel explosive sonic source free ofthe disadvantages associated with conventional explosive sound sources,the sonic source of this invention emitting narrow acoustic beams, eachof a different character in each of a wide variety of directions.

The echo ranging device of this invention is an assembly for producing asequence of sonic pulses which comprises (a) a linear charge ofdetonating explosive arrayed in the form of a helix having a uniformpitch, P, which satisfies the relationship P msxgxt where D is thedetonation velocity of said explosive, L is the length of explosive ineach turn of said helix, and C is the velocity of sound in sea water,said linear charge being so arrayed that the closest approach ofexplosive in successive turns is no less than the minimumcross-sectional dimension of said linear charge; (b) supporting meansadapted to maintain said explosive in said helical array; and (c) aninitiator in initiating relationship to one end of said explosive, saidinitiator being adapted to be actuated by a self-contained actuatingmeans when said assembly is at a predetermined depth in the ocean. Thelinear explosive charge can be arrayed in extended fashion to form thehelical turns as, for example, by taking a length of explosive cord ortape, extended to form essentially a straight line, and wrapping theextended cord or tape to form a helix. Alternatively, the linear chargecan be arrayed in contracted fashion to form the helical turns as, forexample, by bending an explosive cord or tape to form a zigzag, andwrapping the thus contracted cord or tape to form a helix. Whether thelinear charge is extended or contracted, no segment of the charge in anyhelical turn approaches its corresponding segments in adjacent turns bya distance less than the minimum cross-sectional dimension of the linearcharge, i.e., the diameter of a circle if a circular cord is used, orthe thickness of a tape.

Because of its unique structural characteristics, the present echoranging assembly has highly desirable acoustic properties. The explosivecharge of the present assembly is different from conventional charges inthat it possesses all of the following properties: (1) it emitsoscillatory energy, i.e., in the form of a sequence of pressure pulses,instead of a single pulse as emitted by conventional charges; (2) energyis emitted from the charge in substantially equal intensity in a widerange of directions; (3) energy is beamed from the charge in narrowfrequency bands; and (4) the charge is Doppler in character, thefrequency of the pulsation emitted therefrom being angular-dependent.The geometry and detonation velocity of the explosive charge in thepresent assembly can be chosen to provide frequencies which propagatewithout serious attenuation losses in the sea, and which reflect wellfrom a submarine target. In addition, reverberation frequencies can beseparated from echo frequencies because of the Doppler nature of thecharge. That is, for the same echo arrival time, the frequencycharacteristic of an echo from a target located in the near-surfacelayers of the ocean will be difierent from those of the sound reflectedfrom the ocean bottom. Therefore, bottom reflections can be rejected byfiltering the signal. The filters may be chosen so as to pass the bandof frequencies sent toward a submarine target and which will thereforealso be the band of frequencies containing the energy of the echo fromthe target.

The frequency of the sound irradiating the target depends on the angleof elevation between the present assembly and the target. Therefore, thefrequency of the returning echo depends upon the range and depth of thetarget. Since the time of arrival of the echo also depends upon therange of the target, the frequency of the returning echo also dependsupon the time of arrival. Therefore, the filtering process must be timedependent. After the echo of a target is identified, the range to thetarget can be determined by measurement of the time interval betweenemission of the pulse sequence and reception of the echo.

The manner in which the explosive charge present in the assembly of thisinvention provides oscillatory energy of Doppler character is explainedas follows:

When the explosive charge is initiated, a continuous pressure front iscreated which emanates radially from the moving detonation front. For astraight linear charge, the resulting pressure front is conical in form.Because of the helical configuration, this conical pressure front iscoiled up upon itself. Successive portions of this front will thereforearrive in sequence at a fixed point outside the charge. Thus, while thepropagation of detonation from one end of the helical charge to theother is continuous, the helical configuration will cause the pressurepulse emitted in any direction away from and nonaxial to the helix towax and wane as the detonation proceeds in each turn of the helix, thegreatest amplitude being emitted in the chosen direction when thecomponent of the detonation velocity in this direction is equal to thevelocity of sound in water. Therefore, a pressure gauge placed at anyselected position will register the arrival of pressure from successiveturns of the coiled front as a succession of pressure pulses, the timeinterval between the arrival of the successive pulses being determinedby the detonation velocity of the explosive and the length of eachhelical turn. Since the explosive propagates detonation at constantvelocity, the time interval between the arrival of successive pulses ata selected position from the charge will be constant when the pitch andlength of the helical turn are uniform throughout the length of thehelix.

However, inasmuch as the helical configuration introduces an axialdisplacement of the coiled pressure front, the time interval between thearrival of the successive pulses will vary according to the angle ofemanation from the helix. Therefore, the frequency of the pulsation willvary depending upon the angle the emanating pulses make with the axis ofthe helix. When this angle approaches 180 (i.e., a direction along theaxis and in the general direction of the detonation in the helix) thefrequency approaches the maximum level for the charge. When this angleapproaches the frequency approaches the minimum for the same charge. Atany angle between 180? and 0, the wave length and frequency in thespecified direction will be intermediate these values according to theangle of observation.

In order to describe the present invention more fully, reference is nowmade to the accompanying drawings in which FIG. 1 is a front view of onemodification of the sonic-pulseproducing assembly of the presentinvention;

FIG. 2 is a top view of the pulses emanating from the assembly shown inFIG. 1;

FIG. 3 is a schematic view of the pattern of pulses emanating from theassembly shown in FIG. 1;

FIG. 3A shows a series of pulse points along three different paths inthe pattern shown in FIG. 3;

FIG. 4 is a schematic view of a helical explosive charge such as isemployed in the present assembly;

FIGS. 4A through 4E are pressure-time plots for the pulsation arrivingat points A, B, C, D, and E, respectively, of FIG.

FIG. 5 is a front view of another modification of a helical explosivecharge for use in the assembly of this invention;

FIG. 6 shows a series of pulse points obtained from the helical chargeshown in FIG. 5;

FIG. 7 is a front view of still another modification of a helicalexplosive charge for use in the assembly of this invention;

FIG. 8 is a vertical-view diagram showing three targets located in thenear-surface layers of the ocean and at different horizontal distancesfrom the assembly of this invention;

FIG. 9 is a top view of the situation shown in FIG. 8;

FIG. 10 is a top view diagram of one scheme for locating a target by useof the assembly of the present invention; and

FIG. 11 is a display of data obtained after initiation of the helicalexplosive charge of the present assembly in the ocean.

Referring now to the drawings in greater detail in FIG. 1 a length ofexplosive l is wrapped around a hollow tube 2 made of a flexible plasticmaterial, the explosive being wrapped in such a way as to describearound the tube a helix having a pitch, P, which satisfies therelationship C Ps. x-

where D is the detonation velocity of the explosive, L is the length ofexplosive in each helical turn, and C is the velocity of sound in seawater. The pitch of a helix can be broadly defined as the distancebetween helical turns measured parallel to the helical axis. However, inthe present assembly since the helical turns are formed from a linearexplosive charge, each helical turn has a dimension parallel to thehelical axis and the points defining the pitch of the helix must bespecified more precisely. To do this, the periphery of the linearexplosive charge arrayed as a helix can be considered as an envelopehaving an upper boundary line and a lower boundary line, designated Xand Y, respectively, in FIG. 1. As is shown in this figure, the pitch,P, of this helical charge is the distance, measured parallel to thehelical axis, between corresponding points on the upper (or lower)boundary lines of successive turns of the helix. As is seen, the closestapproach of explosive in successive turns of the helix is no less thanthe minimum cross-sectional dimension of the linear charge. The tube 2is constricted at the top by means of a sheet of flexible plasticmaterial having an aperture 7 therein, and the bottom of the tube isheld open by shroud ring 6. A weight 5 hangs from shroud ring 6 by meansof shrouds 4. An initiator, 3, for example, a pressureactuatedinitiator, is attached to the length of explosive at the upper endthereof. The tube is kept fully inflated with water during its descentin the ocean owing to the scooping effect of the open end of the tube.Explosive 1 is initiated when the assembly reaches a depth at which thehydrostatic pressure is sufficient to actuate the initiator 3.

FIG. 2 depicts a portion of the pressure front produced by thedetonation of the helical explosive charge of FIG. 1 as seen lookingdown on the charge. As is shown, the front is continuous and is coiledupon itself as the detonation traverses the helical configuration.

FIG. 3 depicts the plurality of pressure fronts produced by detonationof the helical explosive charge of FIG. 1 as viewed in a plane of theaxis of the charge, the charge having been initiated at a and thedetonation having travelled at a constant velocity through points b, c,d, e, f, g, and h. The pressure fronts, moving at constant velocityradially from the portion of the helix producing them, describe a seriesof eight semicircles having centers at each of the eight points, thesemicircle whose center is a having the longest radius and each of thesemicircles with centers at b, c, d, e, f, g, and h being consecutivelyshorter in radius than the immediately preceding semicircle by aconstant decrement owing to the constant time interval between thedetonation of each point. Because of the downward shift in the centersof the pressure fronts, the distance between fronts is at a minimum inthe direction in which the detonation progresses, and increases to amaximum in the opposite direction. Consequently, three rays A, B, and C,which leave the charge at different angles therefrom, have dilferentfrequencies. A, which is emitted in a downward direction, has a higherfrequency than B, which is emitted in a nearly horizontal direction fromthe charge; and B has a higher frequency than C, which is emitted in anupward direction from the charge.

FIG. 3A illustrates the pulse pattern produced along paths traversed byrays A, B, and C as it would be recorded by a pressure-pulse recorder.

FIGS. 4 and 4A through 4E illustrate the essentially even distributionof sonic energy in a wide range of directions from the assembly of thisinvention. The explosive charge 1 in FIG. 4 is initiated by initiator 3,and the acoustic signal is measured at distant points in the directionsA, B, C, D, and E and at equal distances from the charge. The relativepressures of the signals at equally distant points in the directions A,B, C, D, and E are given in FIGS. 4A, 4B, 4C, 4D, and 4E, respectively.These plots show that in all non'axial directions from the assemblythere is an oscillatory signal. The plots also show the variation of thefrequency of the pulse sequence with the angle of observation.

In FIG. 5, l is a length of explosive arrayed in the form of a helix,the amount of explosive per unit of length of 1 being maximum at thecenter portion of the length of explosive and the amount of explosiveper unit of length decreasing uniformly in both directions from theportion of maximum loading. Because of the difference in explosiveloading, the pulses from the helical charge of FIG. 5 assume the patternshown in FIG. 6.

The helical charge shown in FIG. 7 is preferred for use when it isdesired to emit a pulse sequence in lower frequency ranges where thelonger period requires a long length of explosive per helical turn. Thischarge is constructed from a linear explosive l in the zig-zag form, andthe zigzag charge is arrayed in the form of a helix, the length ofexplosive in the zigzag segment comprising a turn of helix being equalto that in all of the other turns. In considering the pitch, P, of thehelix shown in FIG. 7, the zigzag charge is considered as having animaginary envelope formed by an upper boundary line, X, and a lowerboundary line, Y, indicated by dotted lines in FIG. 7. As is shown, thepitch, P, of this helical charge is the distance, measured parallel tothe helical axis, between corresponding points on the upper (or lower)boundary lines of the envelope of successive turns of the helix. Toprovide a uniform pitch, corresponding bends of the zig-zag are notnecessarily aligned in the helical turns, because the distance betweenXs and Ys in successive turns is uniform.

FIG. 8 shows a vertical cross-section of ocean having a surface S andcontaining the present assembly E, a hydrophone I-I hanging from a floatF, and three target submarines T T and T embedded in the near-surfacelayers of the ocean. H, F, and E lie on approximately the same verticalline. Because T T and T are situated at different distances from E, theywill be in the path of sound of different frequencies. T the targetclosest to E, will be irradiated by sound of frequency f,; T furtherremoved from E, will be irradiated by sound of a higher frequency, f,;and T the furthest target, will be irradiated by sound of still higherfrequency, f

FIG. 9 is a top view of the situation shown in FIG. 8, the float F notbeing shown, however, in order that the assembly E may be depicted. C isa circle drawn in the surface layers of the ocean and designating thosepoints in the oceans surface which are irradiated by sound of frequencyf,. T therefore lies on the circle C Similarly, C and C are circles oflarger radius and designating points irradiated by sound of frequenciesf and f respectively. T and T therefore lie on the circles C and C FIG.10 is a top view of a section of ocean irradiated by the sound producedfrom the assembly E and containing a target T and a hydrophone H, E andH not lying on the same vertical line. C C and C are circles drawn inthe surface layers of the ocean and designating those points in theoceans surface which are irradiated by sounds of frequencies f f and f5,respectively. If an echo of a target is received by the hydrophone H attime t after the explosion of the charge, then the target must liesomewhere on the curve L as well as on cir cle C C or C L represents theintersection of a plane containing the target with an ellipsoid ofrevolution having foci at E and H and equal sound travel times over thepath EPI-I, where P is any point on the surface of the ellipsoid. If thefrequency of the echo of the target is f then the target must also liesomewhere on circle C The circle C and curve L intersect at two points.Therefore, the target must be located at one of these two positions.

A preferred method of displaying the data obtained in echo rangingemploying the present assembly is shown in FIG. 11. The display showsthe series of events occurring when the helical charge of the presentassembly detonates in the direction of the oceans surface and the energyis received on a hydrophone located above the charge in verticalalignment therewith. D identifies the pulse received directly from thecharge; S the reflection from the oceans surface; B the bottomreflection; BS the'bottom-surface reflection; and so on. The hydrophoneoutput is fed to a number of filters, each having a different pass bandwith sharp cutoff frequencies, which separate the output into frequencybands covering the range of frequencies emitted by the charge. The dataare displayed in a variable-density frequency-time presentation, thedensity being proportional to the amplitude of the energy in any givenfrequency band. Since the hydrophone is located above the assembly, thedirect pulse is composed of high-frequency energy which is located at Don the display at maximum frequency and zero time. Reverberations S, SB,etc. which also appear on the upper part of the display, are the resultof specular reflection and scattering from the surface and bottom of theocean of the signal originally emitted by the charge in the upwarddirection. Conversely, pulses which leave the charge in a downwarddirection cause specular reflections and scatterings from the bottom andsurface of the ocean of the signal originally emitted by the charge inthe downward direction, and have their frequency located on the lowerpart of the display. The target echo P is distinguished from biologicalnoise M and N by its predicted frequency content and relatively shortduration, the latter being determined by the emitted pulse length andexpected target size.

Curve F in FIG. 11 is a fiducial line on the presentation which showsthe locus of the expected echo locations, this line being determined bythe frequency distribution of the charge, the two-way travel time of theecho, the charge and hydrophone depths, and refraction effects. Events Qand R represent countermeasure efforts taken by a target submarine. Forevent Q the submarine has recorded the initial pulse and played it backat a later time to give an erroneous range. This event, although it hasthe character of event P, is distinguishable from it by the fact that itdoes not lie on the curve F. Event R is a different countermeasureattempt made by a target submarine. In this case, some time after thesubmarine has heard the direct pulse emitted by the helical charge ithas simulated a broad-band echo of the type produced by conventionalexplosive charges. This countermeasure effort is distinguishable by itsbroad band spectrum.

As is shown in FIGS. 8, 9, and 10, the frequency of the sound emittedtoward the surface layers of the ocean by the detonation of the helicalexplosive charge in the present assembly varies with the distance of theirradiated surface area from the charge. The frequency irradiating thesurface layers increases with time when the charge is initiated at thetop, and this frequency decreases with time when the charge is initiatedat the bottom. Therefore, in an echo ranging method employing thepresent assembly a target may be located by the use of time-dependentselection of the sound recorded from the receiver, i.e., by filteringthe sound so as to separate only certain frequencies at one timeinterval after production of the sonic pulse sequence, and to separatedifferent frequencies at subsequent regular time intervals. Thefrequencies to be separated will be determined by a knowledge of thefrequency characteristics of the sonic pulse sequence produced by thecharge. When an expected frequency is received, the target can belocated by a knowledge of the angle at which such a frequency is emittedby the charge and by measurement of the time interval between emissionof the pulse and reception of the echo. As is seen from FIG. 8, thetarget can be located on a circle when the hydrophone andpulse-producing assembly are positioned on the same vertical line. Inthe embodiment shown in FIG. 10, in which there is horizontaldisplacement between the positions of the hydrophone and the helicalcharge in the ocean, the filtering of the sound with time can be used tolocate the target at two possible positions, i.e., at the twointersections of curve L with the circle designating the echo frequencyf The embodiment shown in FIG. 10 is particularly advantageous in thatit results in a reduction of back-scattered energy which may arrive atthe same time and frequency as the echo pulse when the charge andhydrophone are on the same vertical line.

The echo ranging assembly of this invention contains a linear charge ofdetonating explosive arrayed in the form of a helix whose pitch, P,satisfies the relationship C PSOJSXEXL where D is the detonationvelocity of the explosive, L is the length of explosive in each turn ofthe helix, and C is the velocity of sound in sea water. The pitch, P, isthe axial distance between corresponding points on the upper (or lower)boundary lines at the periphery of the linear charge (or on an imaginaryenvelope enclosing a zig-zag charge) in successive helical turns, asdefined more specifically with reference to FIGS. 1 and 7. The length ofthe turn, L, is the length of explosive connecting these correspondingpoints. The detonation velocity of the explosive used and the length ofeach turn of the helix determine the frequency range of the pulsesequence produced and thus are selected on the basis of the frequenciesdesired. For any combination of D and L, the pitch, P, of the helixsatisfies the above relationship. As a result, the component of thedetonation velocity in the direction of the helical axis is no greaterthan three-fourths the speed of sound in the surrounding medium, i.e.,sea water. This means that the time required for the detonation totravel from one end to the other of the helix is greater than the timerequired for sound in the ocean to travel the length of the axis of thehelix. As a result, the spherical wave front from any segment of thehelical charge will lie inside the wave front generated by anypreviously detonated segment. As the efl'ective axial detonationvelocity approaches the velocity of sound in water, or as P approachesC/DXL, the duration of the pulse emitted in the direction of travel ofthe detonation approaches zero and the amplitude of this pulse becomesvery large. This circumstance is to be avoided, because very strongpulses sent in a vertical direction will rarely produce submarine echoesand will always increase the reverberation interference. In the case inwhich the effective axial detonation velocity exceeds the velocity ofsound in the surrounding water, there will always exist certaindirections from the assembly where the pulses will superimpose to formsuch a single pulse having high energy and a very wide band spectrumwhich is not amenable to the narrow-band filtering technique. Therefore,sonic sources for which P is greater than 0.75 X C/D X L are not usefulwhen narrow-band filtering is employed.

The helical explosive charge used in the assembly of the presentinvention can be made from any detonating explosive which will detonatereliably, when submerged, at a velocity equal to or greater than thevelocity of sound in water, and consequently at a velocity in the rangeof 1,500 to 10,000 meters per second. Illustrative of the types ofexplosive charges which can be used to make the helical charge arePrimacord," comprising a core of explosive surrounded by a nonexplosivesheath; and flexible, self-supporting cords or tapes obtained byextruding an explosive composition. The cross-sectional configuration ofthe length of explosive used in the helical charge is not critical andmay be, for example, circular, semicircular, square, rectangular, oval,or flattened oval. The cord or tape should be capable of reliablysupporting a detonation at the hydrostatic pressure existing at thedesired detonation depth, which will generally be in the range of5-2,000 fathoms.

The required minimum number of turns in the helix will vary according tothe severity of reverberation conditions in a given area, but generallyat least five tums will be necessary. We have found that charges havingabout 50 turns are sufficient for many applications although 5,000 ormore turns may be used in special cases. The required minimum total massof explosive in the charge will generally depend upon the level ofambient noise in the ocean and the required detection capability of thesystem, but will generally be 0.5-l0 pounds.

Each helical turn has the same length of explosive, which will depend onthe frequency range desired and the detonation velocity of theexplosive. If, for example, the explosive detonates at 7,000 meters persecond and a frequency of 10,000 cycles per second is desired in thepulse sequence normal to the helical axis, each turn of the helix mustbe 0.7 meter long. That is, L D/f, where f is the frequency. To providelong helical turns without imposing the necessity of an extremelylarge-diameter helix, the explosive cord or tape from which the helix ismade may be in the form of a zig-zag, as is illustrated in FIG. 7. Allof the bends in the zigzag need not be uniform and corresponding bendsneed not be aligned in the helical turns, but the upper (or lower)boundary lines of the envelope of successive turns must be substantiallyequally spaced to give a uniform pitch. The cord or tape may be providedwith the necessary zig-zag bends to result in the total length ofexplosive desired for a helical turn, and the zig-zag cord or tape maythen be afiixed to the collapsible tube to form a helix in a manner suchthat the length of cord or tape in each turn of the helix is the same.

As stated before, the pitch, or axial distance between correspondingpoints on the upper (or lower) boundaries of the envelopes of successiveturns of the helix will be such as to satisfy the relationship Theparticular pitch used will depend on system requirements such as thespeed, and size of the submarine target, the ambient noise level underwhich the system must operate, the required detection range, and thelimitations on charge size and length imposed by the packagingrequirements.

Because of the reflection characteristics of a submarine and theattenuation of sound in the ocean, the frequency range generally ofinterest in echo ranging is 50 to 20,000 cycles per second. Therefore,the length of the helical turn and the pitch will be such as to providethe desired frequencies in this range when an explosive having adetonation velocity in the abovespecified range is used.

The frequency or period of the pulse sequence emitted at all angles froman assembly of a particular chosen geometry can be determinedexperimentally by measurement, or it can be calculated by the equation:

where T is the period of the pulse sequence emanating in a normaldirection from the assembly and is equal to L/D (L is the length ofexplosive in a helical turn and D is the detonation velocity of theexplosive), P is the pitch of the helix, C is the velocity of sound inthe sea, and T is the period of the pulse sequence emanating at someangle 4). (b is the angle between the direction from the array to anobserver and the axial direction away from which the detonation travels.

The obtaining of the desired pulse sequence is dependent on thecontinuous propagation of detonation throughout all of the turns of thehelical charge. Such continuous propagation will not be obtained if thedetonation is propagated across the space between turns or if thedetonation of a turn is physically disrupted by that of a previous turn.The distance between helical turns which will prevent continuouspropagation will depend on the size of the helical charge and the formof the cord cross-section as well as on the sensitivity of theexplosive. Generally, however, the charge should be so arrayed that theclosest approach of explosive in successive turns of the helix is noless than the minimum cross-sectional dimension (S) of the linearcharge. Usually S will be at least 1 millimeter. For the embodimentshown in FIG. 1, this spacing places an additional requirement on thepitch, P, i.e., that P should be no less than twice the minimumcross-sectional dimension of the linear charge. For the embodiment shownin FIG. 7, P should be no less than the total of (l) the minimumcross-sectional dimension of the linear explosive charge and (2) thewidth of the envelope bounding the zig-zag measured in an axialdirection.

We have found that a preferred wave form and frequency spectrum can beobtained when the helical charge is designed in such manner that theamplitude of the pulses in a sequence is not constant. Such variation inamplitude can most readily be achieved by having the amount of explosiveper unit of length varied for difierent turns of the helix. Aparticularly desirable arrangement is one which will provide a sequenceof pulses which vary in amplitude according to a Gaussian distribution,i.e., the amplitude of each successive pulse is increased uniformly to amaximum and thereafter decreased uniformly to the level of the firstpulse. Such pulse emission will provide a reduction in side-lobe energyin a given frequency band. The foregoing can be achieved by forming thehelix from a length of explosive in which the amount of explosive perunit of length is uniformly or step-wise varied from a minimum at eachend to a maximum at the center. The rate of variation will be dependentupon the number of turns in the helix, the minimum amount of explosiveper unit of length which will provide a pulse of sufficient amplitudefor detection, and the maximum amount of explosive per unit of lengththat can be tolerated in the helix configuration. Further, a helicalarray of many turns may consist of a series of units of varyingexplosive loading.

Various types of support means can be used to maintain the desiredhelical configuration. In one embodiment, the tube 2 of FIG. 1 may be arigid cardboard tube having the explosive charge attached to it by anyconvenient means, e.g., by stapling or by fitting into grooves in thecardboard. The lower end of the tube will be weighted to assure avertical fall of the assembly when dropped into the water.

Inasmuch as the handling and transportation of such an assembly mayprove difficult particularly when the helical charge is a large one, aconvenient method of packaging the charge is to fashion tube 2 of FIG. 1from a flexible plastic material, e.g., polyethylene. In this case,after affixing of the explosive charge onto the plastic, the tube willbe collapsed for easy handling. The tube will be constricted at itsupper end, and the lower end will be weighted. When the assembly isdropped into the water, the entry of water into the tube will open thetube into its expanded position.

In an alternate packaging means, the tube 2 of FIG. 1 may be eliminated,and the helical charge may be affixed to a wire of spring steel of thedesired pitch.

The detonation of the helical explosive charge is initiated by aninitiator attached to one end of the charge. The means for actuating theinitiator is self-contained in the assembly and requires no connectionwith a vessel. Various initiators of this kind are known, for example,those which are actuated by the pressure of the water when a preselecteddepth is reached, e.g., the initiator described in U.S. Pat. No.2,726,602. Alternatively, a timing device may be used as the actuatingmeans.

The end from which the charge is initiated is optional, both bottomandtop-initiation providing the necessary Doppler effect.

Although the utility of the present assembly has been illustrated forecho ranging systems in which there is a vertical separation between thehelical explosive charge and the expected depth of the target, theassembly can also be used in situations in which the charge is placedshallow in the ocean, i.e., at approximately the same depth as anexpected target. In such situations, the system of time-dependentselection of the frequencies is effective and, in addition, if thehorizontal distance between the charge and the target is not great, thevariation in frequency will provide a determination of the depth of thetarget. However, at long ranges the frequency beam becomes sufficientlybroad so that very little indication of depth of the target can bedetected.

The time-dependent selection which may be used to advantage inconjunction with the assembly of this invention to determine thelocation of a submarine is also advantageous in that it provides acertain degree of immunity to countermeasures which might be taken bythe submarine to escape detection. In conventional systems, the locatingof the submarine is dependent on an accurate measure of the timeinterval between the reception of the direct pulse from the sonic sourceand the echo pulse from the reflected object, this time intervalaffording a measure of the additional distance travelled by the echopulse over the distance between the sensing device and the source. Thesubmarine may interfere with such a system by sending out a pulse of thesame frequency as that of the source. However, as is shown in FIG. 11,with the present assembly such a countermeasure would be ineffectiveinasmuch as it is used in conjunction with a time-dependent selection offrequencies, i.e., the expected frequency of the echo being received bythe sensing device will change with time. Thus, the signal sent out bythe submarine can be recog nized as countermeasure noise because it willgenerally have the wrong frequency for its time of arrival.

Another advantageous feature of using the present assembly in a methodof echo ranging over conventional methods is that, owing to thetime-dependent selection of the frequencies, several of these assembliesmay be employed at the same time without interfering with each other.

The invention has been described in detail in the foregoing. Manymodifications will be apparent to those skilled in the art and will notrequire departure from the spirit of this invention. Accordingly, weintend to be limited only by the following claims.

We claim:

1. A sonic-pulse-producing assembly for use in underwater echo rangingwhich comprises a. a linear charge of detonating explosive arrayed inthe form of a helix having a uniform pitch, P, which satisfies therelationship C PsOf/Sx-x DA L where D is the detonation velocity of saidexplosive, L is the length of explosive in each turn of said helix, andC is the velocity of sound in sea water, said linear charge being soarrayed that the closest approach of explosive in successive turns ofsaid helix is no less than the minimum cross-sectional dimension of saidlinear charge;

b. supporting means adapted to maintain said explosive in said helicalarray; and

c. an initiator in initiating relationship to one end of said explosive,said initiator being adapted to be actuated by a self-containedactuating means when said assembly is at a predetermined depth in theocean.

2. An assembly according to claim 1, wherein said initiator is actuatedby a means responsive to hydrostatic pressure.

3. An assembly for producing a sequence of pressure pulses whosefrequency varies in different directions from said assembly whichcomprises a. a linear charge of detonating explosive arrayed in the formof a helix having a uniform pitch, P, which satisfies the relationshipwhere D is the detonation velocity of said explosive and lies within therange of 1,500 to 10,000 meters per second, C is the velocity of soundin sea water, and L is the length of explosive in each turn of saidhelix and is equal to D/f, where D is as defined above and f is afrequency within the range of 50 to 20,000 cycles per second, saidlinear charge being so arrayed that the closest approach of explosive insuccessive turns of said helix is no less than the minimumcross-sectional dimension of said linear charge;

b. supporting means adapted to maintain said explosive in said helicalarray; and c. an initiator in initiating relationship to one end of saidexplosive, said initiator being adapted to be actuated by aself-contained actuating means when said assembly is at a predetermineddepth in the ocean. 4. A sonic-pulse-producing assembly for use inunderwater echo ranging which comprises a. a collapsible tube of plasticfilm and having one constricted end and one fully open end;

b. a linear charge of detonating explosive affixed to said tube in amanner such as to fonn a helix having a uniform pitch, P, whichsatisfies the relationship where D is the detonation velocity of saidexplosive, L is the length of explosive in each turn of said helix, andC is the velocity of sound in sea water, said linear charge being soarrayed that the closest approach of explosive in successive turns ofsaid helix is no less than the minimum cross-sectional dimension of saidlinear charge;

0. an initiator in initiating relationship to one end of said explosive,said initiator being adapted to be actuated by a self-containedactuating means when said assembly is at a predetermined depth in theocean; and

d. a weight attached to said open end of said collapsible tube.

5. An assembly for producing a sequence of pressure pulses whosefrequency varies in different directions from said assembly whichcomprises a. a linear charge of detonating explosive bent in the form of12 a zig-zag, said zig-zag explosive charge being arrayed in the form ofa helix having a uniform pitch, P, which satisfies the relationshipwhere D is the detonation velocity of said explosive and lies within therange of 1,500 to 10,000 meters per second, C is velocity of sound insea water, and L is the length of explosive in each turn of said helixand is equal to D/f, where D is as defined above and f is a frequencywithin the range of 50 to 20,000 cycles per second, said zig-zag chargebeing so arrayed that the closest approach of explosive in successiveturns of said helix is no less than the minimum cross-sectionaldimension of said linear charge;

b. supporting means adapted to maintain said explosive in said helicalarray; and c. an initiator in initiating relationship to one end of saidexplosive, said initiator being adapted to be actuated by aself-contained actuating means when said assembly is at a predetermineddepth in the ocean.

1. A sonic-pulse-producing assembly for use in underwater echo rangingwhich comprises a. a linear charge of detonating explosive arrayed inthe form of a helix having a uniform pitch, P, which satisfies therelationship where D is the detonation velocity of said explosive, L isthe length of explosive in each turn of said helix, and C is thevelocity of sound in sea water, said linear charge being so arrayed thatthe closest approach of explosive in successive turns of said helix isno less than the minimum cross-sectional dimension of said linearcharge; b. supporting means adapted to maintain said explosive in saidhelical array; and c. an initiator in initiating relationship to one endof said explosive, said initiator being adapted to be actuated by aself-contained actuating means when said assembly is at a predetermineddepth in the ocean.
 2. An assembly according to claim 1, wherein saidinitiator is actuated by a means responsive to hydrostatic pressure. 3.An assembly for producing a sequence of pressure pulses whose frequencyvaries in different directions from said assembly which comprises a. alinear charge of detonating explosive arrayed in the form of a helixhaving a uniform pitch, P, which satisfies the relationship where D isthe detonation velocity of said explosive and lies within the range of1,500 to 10,000 meters per second, C is the velocity of sound in seawater, and L is the length of explosive in each turn of said helix andis equal to D/f, where D is as defined above and f is a frequency withinthe range of 50 to 20, 000 cycles per second, said linear charge beingso arrayed that the closest approach of explosive in successive turns ofsaid helix is no less than the minimum cross-sectional dimension of saidlinear charge; b. supporting means adapted to maintain said explosive insaid helical array; and c. an initiator in initiating relationship toone end of said explosive, said initiator being adapted to be actuatedby a self-contained actuating means whEn said assembly is at apredetermined depth in the ocean.
 4. A sonic-pulse-producing assemblyfor use in underwater echo ranging which comprises a. a collapsible tubeof plastic film and having one constricted end and one fully open end;b. a linear charge of detonating explosive affixed to said tube in amanner such as to form a helix having a uniform pitch, P, whichsatisfies the relationship where D is the detonation velocity of saidexplosive, L is the length of explosive in each turn of said helix, andC is the velocity of sound in sea water, said linear charge being soarrayed that the closest approach of explosive in successive turns ofsaid helix is no less than the minimum cross-sectional dimension of saidlinear charge; c. an initiator in initiating relationship to one end ofsaid explosive, said initiator being adapted to be actuated by aself-contained actuating means when said assembly is at a predetermineddepth in the ocean; and d. a weight attached to said open end of saidcollapsible tube.
 5. An assembly for producing a sequence of pressurepulses whose frequency varies in different directions from said assemblywhich comprises a. a linear charge of detonating explosive bent in theform of a zig-zag, said zig-zag explosive charge being arrayed in theform of a helix having a uniform pitch, P, which satisfies therelationship where D is the detonation velocity of said explosive andlies within the range of 1,500 to 10,000 meters per second, C isvelocity of sound in sea water, and L is the length of explosive in eachturn of said helix and is equal to D/f, where D is as defined above andf is a frequency within the range of 50 to 20, 000 cycles per second,said zig-zag charge being so arrayed that the closest approach ofexplosive in successive turns of said helix is no less than the minimumcross-sectional dimension of said linear charge; b. supporting meansadapted to maintain said explosive in said helical array; and c. aninitiator in initiating relationship to one end of said explosive, saidinitiator being adapted to be actuated by a self-contained actuatingmeans when said assembly is at a predetermined depth in the ocean.